The present invention relates generally to polyurethane-based pressure-sensitive adhesives, systems (e.g., 100% solids, waterborne, and solventborne) for such adhesives, articles therefrom, and methods of their preparation.
A wide variety of polyurethane-based adhesives are known. For example, see U.S. Pat. No. 5,910,536 (Kydonieus et al.), which describes a particular type of polyurethane-based adhesivexe2x80x94a pressure-sensitive poly[urethane-(meth)acrylate]-based adhesive. The adhesives therein are prepared from 100% solids (i.e., essentially solvent-free and water-free) systems.
In addition to 100% solids systems, it is known to produce polyurethane-based adhesives in both solventborne (i.e., those using mostly organic solvents as a solvating medium) and waterborne (i.e., those using mostly water as a dispersing medium) systems. These adhesive systems are applied to a substrate in the form of a solution or dispersion, respectively. Generally, whether the adhesive system is solventborne or waterborne, it must be coated onto a desired substrate and dried to remove solvating or dispersing medium (i.e., organic solvent or water, respectively) in order to form an adhesive coating.
Reactive polyurethane-based adhesive systems have been described in the literature, but coatable mixtures prepared from such systems generally have a limited useful life. Examples of reactive polyurethane-based adhesive systems include those described in U.S. Pat. No. 3,246,049 (Webber); U.S. Pat. No. 3,437,622 (Dahl); U.S. Pat. No. 3,718,712 (Tushaus); U.S. Pat. No. 3,879,248 (Kest); U.S. Pat. No. 3,925,283 (Dahl); U.S. Pat. No. 4,087,392 (Hartmann); U.S. Pat. No. 5,102,714 (Mobley et al.); U.S. Pat. No. 5,486,570 (St. Clair); U.S. Pat. No. 5,591,820 (Kydonieus); and U.S. Pat. No. 5,714,543 (Shah). Further descriptions include those in GB 1,113,925 (Weller); GB 1,216,672 (Grindley); and PCT Publication Number WO 97/22,642 (Chang).
In reactive systems, typically multiple parts must be mixed to form a coatable reacting mixture. The reacting mixture must then be coated onto a substrate within a short period of time. If the reacting mixture is not coated within a short period of time, the viscosity of the composition will become too high, rendering the composition uncoatable. Thus, storage-stable, coatable adhesive systems are not obtainable when the adhesive system is a reactive system.
In addition to not being storage-stable, there are other disadvantages associated with reactive systems. Typically the parts of a reactive polyurethane-based adhesive system include an isocyanate-containing part (i.e., an isocyanate-terminated polyurethane prepolymer) and a chain extending part. Due to the presence of isocyanate-functional groups on the polyurethane prepolymer, storage of that part must be carefully controlled so that moisture does not react with the isocyanate-functional groups, rendering the composition non-reactive and, thus, unusable. Sensitivity to moisture can also lead to variations in properties of these coated adhesives due to, for example, local variations in ambient temperature and humidity when the adhesive is coated. Furthermore, special handling procedures may be required for the multi-part system, especially by those that are sensitive to isocyanate chemicals.
Thus, essentially non-reactive systems are preferable over reactive systems from at least a storage stability standpoint and ease of use standpoint. Premixing of components and special storage considerations for the components are not required in non-reactive systems.
When using a non-reactive, solventborne or waterborne system, to form an adhesive coating on a substrate, one merely applies the composition, which contains a fully reacted polymer in the form of a solution or dispersion, to the substrate and then dries the solvating or dispersing medium to form the adhesive coating. However, such non-reactive systems may require the addition of external emulsifiers or cationic stabilization agents to maintain stability of the solution or dispersion prior to coating to form the adhesive.
Many polyurethane-based and polyurethane-urea-based dispersions are known in the literature. For example, see U.S. Pat. No. 5,037,864 (Anand et al.); U.S. Pat. No. 5,354,807 (Dochniak); U.S. Pat. No. 5,354,808 (Onwumere et al.); U.S. Pat. No. 5,554,686 (Frisch, Jr. et al.); U.S. Pat. No. 5,608,000 (Duan); U.S. Pat. No. 5,807,919 (Duan); and U.S. Pat. No. 5,863,980 (Choi et al.) as well as JP-07-102,233 (Sekisui Chemical). Most of the literature references, however, do not describe how to prepare pressure-sensitive adhesive (PSA)s from the dispersions.
PSA compositions are a unique subset of adhesives well known to those of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power.
U.S. Pat. No. 5,910,536 (Kydonieus et al.), described supra, describes how a xe2x80x9csuitable balance of elastic and viscous properties which is required in pressure-sensitive adhesives has not been readily attainable in conventional polyurethane materials.xe2x80x9d Although Kydonieus et al. did not teach polyurethane-based PSA dispersions, they were able to obtain polyurethane-based PSAs from a 100% solids system. Yet, in order to obtain the balance of properties requisite to PSAS, the polyurethane-based PSAs described therein were poly[urethane-(meth)acrylate]-based, containing both urethane and acrylate linkages in their polymeric backbone. However, as recognized by Kydonieus et al., disadvantages with these types of polymers include the fact that acrylic-based adhesives are not as strong as polyurethane-based adhesives. Furthermore, acrylic-based adhesives, such as those described by Kydonieus et al., are generally more irritating to human skin than polyurethane-based adhesives.
Certain few references do describe preparation of PSAs from polyurethane-based dispersions. For example, PCT Publication Number WO 98/31,760 (Dow Chemical Company) describes a polyurethane PSA-forming latex composition comprising a polyurethane PSA-forming material, wherein a polyurethane PSA is obtained by dehydrating the PSA-forming latex composition. The process for preparing the polyurethane PSA includes emulsifying a polyurethane prepolymer in water, chemically reacting the prepolymer to react substantially all unreacted functional groups, and dehydrating the aqueous dispersion. The polyurethane prepolymer is formed from reactants including polyisocyanates and active hydrogen compounds, such as polyols. It is stated that polyols can be employed individually or in mixtures as di-, or a combination thereof, polyoxyalkylene polyols. Suitable active hydrogen compounds are polyols having a molecular weight less than 6,000.
Also see U.S. Pat. No. 3,796,678 (Bartizal), where highly branched, capped polyurethane dispersions for preparing PSAs are described. It is stated that at least about 20 weight percent pendant chains extend from the polyurethane and polyurethane-urea polymers therein. The polymers are formed in and dispersed in an aqueous medium.
Still further polyurethane-based chemistries for preparing PSAs are desirable. It would be particularly desirable to provide polyurethane-based PSAs that can be tailored to have a wide range of peel adhesion and shear strength properties.
Polyurethane-based pressure-sensitive adhesives (PSAs) of the invention comprise the reaction product of: a polyol component comprising at least one diol having a weight average molecular weight of at least about 2,000, wherein the at least one diol comprises less than about 8 weight % monols; an isocyanate-functional component; an optional reactive emulsifying compound; and an optional chain extending agent. The PSAs can be prepared from 100% solids, waterborne or solventborne systems. For example, the PSAs can be prepared from poly(urethane-urea) dispersions.
As a further example, a polyurethane-based PSA dispersion of the invention comprises the reaction product of: a polyol component comprising at least one diol having a weight average molecular weight of at least about 2,000, wherein the at least one diol comprises less than about 8 weight % monols; an isocyanate-functional component; an optional reactive emulsifying compound; and an optional chain extending agent, wherein the reaction product is dispersed in a dispersing medium. A polyurethane-based PSA solution of the invention comprises the reaction product of: a polyol component comprising at least one diol having a weight average molecular weight of at least about 2,000, wherein the at least one diol comprises less than about 8 weight % monols; an isocyanate-functional component; an optional reactive emulsifying compound; and an optional chain extending agent, wherein the reaction product is solvated in a solvating medium.
In one embodiment, the polyol component comprises at least one polyoxyalkylene polyol. In another embodiment, each polyol in the polyol component is a diol. In another embodiment, the at least one diol comprises a diol having a ratio of diol molecular weight to weight % monol of at least about 800, or still further, at least about 1,000, or even further, at least about 1,500.
In further embodiments of the invention, the polyol component comprises a first polyol having a weight average molecular weight of less than 2,000 and a second polyol having a weight average molecular weight of 2,000 or more. In further embodiments, the first polyol has a weight average molecular weight of less than about 1,800. In yet further embodiments, the first polyol has a weight average molecular weight of less than about 1,000.
Similarly, in further embodiments of the invention, the second polyol has a weight average molecular weight of greater than about 2,500. In even further embodiments, the second polyol has a weight average molecular weight of greater than about 6,000.
When viewed from a different angle, in a further embodiment of the invention, the weight average molecular weights of the first and second polyols differ by at least about 500. In even further embodiments, the weight average molecular weights of the first and second isocyanate-reactive materials differ by at least about 2,500.
According to one aspect of this embodiment, the second polyol comprises a majority of the polyol component based on total weight of the polyol component. For example, in one embodiment, the first polyol comprises about 1 to about 40 percent by weight of the polyol component and the second polyol comprises about 60 to about 99 percent by weight of the polyol component. In yet another embodiment, the first polyol comprises about 5 to about 25 percent by weight of the polyol component and the second polyol comprises about 75 to about 95 percent by weight of the polyol component.
In one embodiment, the isocyanate-functional component comprises a diisocyanate. In one embodiment, the reactive emulsifying compound comprises at least about 0.5% by weight of the total reactants. In another embodiment, the polyurethane-based PSA further comprises the reaction product of a chain extending agent.
PSAs of the invention may be at least partially coated on a substrate. For example, PSAs of the invention are useful in tapes. The tapes comprise a backing having a first and second side and the PSA coated on at least a portion of the first side of the backing and, optionally, on at least a portion of the second side of the backing.
A method of preparing the polyurethane-based PSAs of the invention comprises the, not necessarily sequential, steps of: providing a polyol component comprising at least one diol having a weight average molecular weight of at least about 2,000 wherein the at least one diol comprises less than about 8 weight % monols; providing an isocyanate-functional component; optionally providing a reactive emulsifying compound; allowing the polyol component, the isocyanate-functional component, and the optional reactive emulsifying compound to react to form a polyurethane prepolymer; and chain extending the polyurethane prepolymer. According to further embodiments, the method can further comprise the step of dispersing the polyurethane prepolymer in a dispersing medium. In still further embodiments, the method can further comprise the step of drying the dispersing medium to form a coating of the polyurethane-based PSA.
Pressure-sensitive adhesives (PSAs) of the invention are polyurethane-based. For simplicity, the term xe2x80x9cpolyurethanexe2x80x9d as used herein includes polymers containing urethane (also known as carbamate) linkages, urea linkages, or combinations thereof (i.e., in the case of poly(urethane-urea)s). Thus, polyurethane-based PSAs of the invention contain at least urethane linkages and, optionally, urea linkages. Furthermore, PSAs of the invention are based on polymers where the backbone has at least 80% urethane and/or urea repeat linkages formed during the polymerization process, such as the polymerization processes described below. That is, the polyurethane-based polymers are formed from prepolymers that are preferably terminated by isocyanate groups. Then, further reactants used to form the PSAs from the prepolymers are selected such that no more than about 20%, preferably no more than about 10%, more preferably no more than about 5%, and preferably none of the repeat linkages between polymeric segments formed in the polymeric backbone during polymerization are other than urethane and urea linkages.
PSAs of the invention are preferably prepared from systems that are essentially non-reactive. Furthermore, polyurethane-based PSA systems of the invention are preferably storage-stable. xe2x80x9cStorage-stablexe2x80x9d PSA systems are those compositions that can be coated on a substrate to form a continuous film at any time after the composition is formed up until the shelf life of the material has expired. Preferably, the shelf life of the material is at least three days, more preferably at least about one month, even more preferably at least about six months, and most preferably at least about one year.
PSAs of the present invention may be derived from 100% solids, solventborne or waterborne systems. Environmental and regulatory demands are prompting manufacturers of adhesives to move more rapidly from solventborne systems to waterborne systems. As compared to organic solvents, water is less costly and more environmental friendly. Furthermore, flammability and combustibility of waterbome systems is reduced as compared to solventborne systems. Thus, it is preferred that the polyurethane-based PSAs of the invention are derived from waterborne systems, using essentially only water as the dispersing medium.
Dispersions of the invention are prepared by reacting components, including at least one isocyanate-reactive (e.g., hydroxy-functional, such as polyol) component, at least one isocyanate-functional (e.g., polyisocyanate) component, and, optionally, at least one reactive emulsifying compound, to form an isocyanate-terminated polyurethane prepolymer. The polyurethane prepolymer is then dispersed, and chain-extended, in a dispersing medium to form polyurethane-based dispersions of the invention.
Components of polyurethane-based PSAs of the invention are further described below, with reference to certain terms understood by those in the chemical arts as referring to certain hydrocarbon groups. Reference is also made throughout the specification to polymeric versions thereof. In that case, the prefix xe2x80x9cpolyxe2x80x9d is inserted in front of the name of the corresponding hydrocarbon group.
Except where otherwise noted, such hydrocarbon groups, as used herein, may include one or more heteroatoms (e.g., oxygen, nitrogen, sulfur, or halogen atoms), as well as functional groups (e.g., oxime, ester, carbonate, amide, ether, urethane, urea, carbonyl groups, or mixtures thereof).
The term xe2x80x9caliphatic groupxe2x80x9d means a saturated or unsaturated, linear, branched, or cyclic hydrocarbon group. This term is used to encompass alkylene (e.g., oxyalkylene), aralkylene, and cycloalkylene groups, for example.
The term xe2x80x9calkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group. Particularly preferred alkylene groups are oxyalkylene groups.
The term xe2x80x9coxyalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group with a terminal oxygen atom.
The term xe2x80x9caralkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group containing at least one aromatic group.
The term xe2x80x9ccycloalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group containing at least one cyclic group.
The term xe2x80x9coxycycloalkylene groupxe2x80x9d means a saturated, linear or branched, divalent hydrocarbon group containing at least one cyclic group and a terminal oxygen atom.
The term xe2x80x9caromatic groupxe2x80x9d means a mononuclear aromatic hydrocarbon group or polynuclear aromatic hydrocarbon group. The term includes arylene groups.
The term xe2x80x9carylene groupxe2x80x9d means a divalent aromatic group.
Isocyanate-Reactive Component
Any suitable isocyanate-reactive component can be used in the present invention. The isocyanate-reactive component contains at least one isocyanate-reactive material or mixtures thereof As understood by one of ordinary skill in the art, an isocyanate-reactive material includes at least one active hydrogen. Those of ordinary skill in the polyurethane chemistry art will understand that a wide variety of materials are suitable for this component. For example, amines, thiols, and polyols are isocyanate-reactive materials.
However, it is preferred that the isocyanate-reactive material be a hydroxy-functional material. Polyols are the preferred hydroxy-functional material used in the present invention. Polyols of the invention can be of any molecular weight, including relatively low molecular weight polyols (i.e., having a weight average molecular weight of less than about 250) commonly referred to as xe2x80x9cchain extendersxe2x80x9d or xe2x80x9cchain extending agents,xe2x80x9d as well as those polyols having higher molecular weights. Polyols provide urethane linkages when reacted with an isocyanate-functional component, such as a polyisocyanate.
Polyols, as opposed to monols, have at least two hydroxy-functional groups. Generally and preferably, diols are used in the present invention. Diols contribute to formation of relatively high molecular weight polymers without requiring crosslinking, such as is conventionally introduced by polyols having greater than two hydroxy-functional groups. PSAs prepared from such diols generally have increased shear strength, peel adhesion, and/or a balance thereof, to provide PSA properties that may be desired for certain applications. In contrast, polymers having a relatively large amount of crosslinking may not be suitable for many PSA applications and/or materials therefrom may not be readily processable.
Examples of polyols useful in the present invention include, but are not limited to, polyester polyols (e.g., lactone polyols) and the alkylene oxide (e.g., ethylene oxide; 1,2-epoxypropane; 1,2-epoxybutane; 2,3-epoxybutane; isobutylene oxide; and epichlorohydrin) adducts thereof, polyether polyols (e.g., polyoxyalkylene polyols, such as polypropylene oxide polyols, polyethylene oxide polyols, polypropylene oxide polyethylene oxide copolymer polyols, and polyoxytetramethylene polyols; polyoxycycloalkylene polyols; polythioethers; and alkylene oxide adducts thereof), polyalkylene polyols, mixtures thereof, and copolymers therefrom. Polyoxyalkylene polyols are preferred.
When copolymers are used, chemically similar repeating units may be randomly distributed throughout the copolymer or in the form of blocks in the copolymer. Similarly, chemically similar repeating units may be arranged in any suitable order within the copolymer. For example, oxyalkylene repeating units may be internal or terminal units within a copolymer. The oxyalkylene repeating units may be randomly distributed or in the form of blocks within a copolymer. One preferred example of a copolymer containing oxyalkylene repeating units is a polyoxyalkylene-capped polyoxyalkylene polyol (e.g., a polyoxyethylene-capped polyoxypropylene).
Certain applications will benefit from using PSAs having fewer residuals (i.e., reactive components, such as monomers, that remain unreacted in the reaction product) than conventional PSAs. Such applications include, for example, electronics applications and medical applications. The presence of residuals in PSAs used for these applications may be problematic. For example, the presence of residuals in PSAs used for electronics applications may contaminate other components in the electronic component, for example, by acting as a plasticizer. Plasticization of magnetic media in a hard disk drive could result in a shortened useful life for the hard disk drive. The presence of residuals in PSAs used for medical applications may cause irritation, sensitization, or skin trauma if the residuals migrate from the PSA to the surface in contact with skin, for example. This problem was recognized by Kydonieus et al., in U.S. Pat. No. 5,910,536, as being associated with acrylate-based adhesives. It is also foreseeable that PSAs having fewer residuals will be advantageous for use in applications having contact with, for example, food or other products for human consumption.
When higher molecular weight polyols (i.e., polyols having weight average molecular weights of at least about 2,000) are used, it is preferred that the polyol component be xe2x80x9chighly purexe2x80x9d (i.e., the polyol approaches its theoretical functionalityxe2x80x94e.g., 2.0 for diols, 3.0 for triols, etc.). These highly pure polyols preferably have a ratio of polyol molecular weight to weight % monol of at least about 800, preferably at least about 1,000, and more preferably at least about 1,500. For example, a 12,000 molecular weight polyol with 8 weight % monol has such a ratio of 1,500 (i.e., 12,000/8=1,500). Preferably, the highly pure polyol contains about 8% by weight monol or less.
Generally, as the molecular weight of the polyol increases in this preferred embodiment, a higher proportion of monol may be present in the polyol. For example, polyols having molecular weights of about 3,000 or less preferably contain less than about 1% by weight of monols. Polyols having molecular weights of greater than about 3,000 to about 4,000 preferably contain less than about 3% by weight of monols. Polyols having molecular weights of greater than about 4,000 to about 8,000 preferably contain less than about 6% by weight of monols. Polyols having molecular weights of greater than about 8,000 to about 12,000 preferably contain less than about 8% by weight of monols.
Examples of highly pure polyols include those available from Lyondell Chemical Company of Houston, Texas, under the trade designation, ACCLAIM and certain of those under the trade designation, ARCOL. To determine whether these preferred highly pure polyols were used in preparing PSAs of the invention, Nuclear Magnetic Resonance (NMR) or other suitable analytical methods can be used.
Other benefits derived from using highly pure polyols include the ability to form relatively high molecular weight polymers without requiring crosslinking, such as is conventionally introduced into polymers by polyols having greater than two hydroxy-functional groups. In comparison, for example, when conventional diols (e.g., those diols having greater than about 10% by weight or greater of monols) are used to prepare polyurethanes, higher functional polyols (e.g., triols) are also typically used in an attempt to balance the stoichiometric ratio of isocyanate-reactive (e.g., hydroxy-functional) groups to isocyanate-functional groups in the reaction mixture. It is the higher-functional polyols (i.e., those having more than two hydroxy-functional groups) that predominantly contribute to crosslinking of the polymer.
In general, preferred diols useful in the present invention can be represented by Formula I: 
wherein R represents an aliphatic group, aromatic group, mixtures thereof, polymers thereof, or copolymers thereof. Preferably R is a polyalkylene group, polyoxyalkylene group, or mixtures thereof
Although polyols containing more than two hydroxy-functional groups are generally less preferred than diols, certain higher functional polyols may also be used in the present invention. These higher functional polyols may be used alone, or in combination with other isocyanate-reactive materials, for the isocyanate-reactive component.
In one aspect of this embodiment, these higher functional polyols are converted to diols prior to their use in the isocyanate-reactive component. After conversion, the reaction products are considered diols according to the present invention. For example, one preferred class of higher functional polyols that can be used in the present invention includes polyoxyalkylene triols, which can be reacted with a carboxylic acid cyclic anhydride or a sulfocarboxylic acid cyclic anhydride to reduce the functionality thereof The polyoxyalkylene triol is preferably polyoxypropylene or, more preferably, a polyoxypropylene polyoxyethylene copolymer. The cyclic carboxylic anhydride is preferably selected from anhydrides such as succinic; glutaric; cyclohexanedicarboxylic; methylsuccinic; hexahydro-4-methylphthalic; phthalic; 1,2,4-benzenetricarboxylic; maleic; fumaric; itaconic; 3,4,5,6-tetrahydrophthalic; 1-dodecen-1-yl succinic; cis-aconitic; and mixtures thereof. The sulfocarboxylic cyclic anhydride is preferably 2-sulfobenzoic acid cyclic anhydride.
When the triol molecular weight used to prepare such reaction products is relatively low (i.e., having a weight average molecular weight of less than 3,000), the ester-acid reaction products are preferably used in combination with another isocyanate-reactive material. The use of such lower molecular weight triols in combination with another isocyanate-reactive material may even obviate the need for the reactive emulsifying compound, which is described below, when preparing polyurethane-based dispersions of the invention. When the triols have a molecular weight of 3,000 or greater, preferably 4,500 or greater, the ester-acid reaction product is generally suitable for use without other isocyanate-reactive materials and may also obviate the need for the later described reactive emulsifying compound when preparing polyurethane-based dispersions of the invention.
For broader formulation latitude, at least two isocyanate-reactive materials, such as polyols, may be used for the isocyanate-reactive component. It has been found that using at least one material having a relatively low weight average molecular weight in combination with at least one material having a relatively high weight average molecular weight results in PSAs having significantly greater shear strength (i.e., holding power), but comparable, or still adequate, peel adhesion, as compared to those PSAs derived from isocyanate-reactive components containing a single isocyanate-reactive material. Thus, this aspect of the present invention provides PSAs that can be used in applications where higher holding power is desired, but ease of removability from the adherend is also desired. However, the ratio and types of materials in the isocyanate-reactive component mixture can be adjusted to obtain a wide range of shear strengths and peel adhesions in PSAs prepared therefrom.
The use of a mixture of materials for the isocyanate-reactive component also allows for improved cost-effectiveness, where desired. For example, more expensive isocyanate-reactive materials may be mixed with less expensive isocyanate-reactive materials for the isocyanate-reactive component. This may be the case, for example, when highly pure relatively high molecular weight polyols are mixed with relatively high molecular weight polyols that are less pure or with lower molecular weight polyols.
Preferably, the mixture of isocyanate-reactive materials for the isocyanate-reactive component includes at least two diols, most preferably all polyols in the mixture are diols. As stated above, diols are preferred over other polyols due to their tendency to contribute to higher molecular weight polymers without crosslinking. Thus, if present, it is preferred that polyols other than diols comprise less than about 10% by weight, more preferably less than about 5% by weight, of the polyol component.
When using a combination of a relatively low weight average molecular weight polyol and a relatively high weight average molecular weight polyol, it is preferred that the weight average molecular weights of the two polyols differ by at least about 500, more preferably at least about 1,000, even more preferably at least about 1,500, even more preferably at least about 2,000, and most preferably at least about 2,500. For purposes of this patent, relatively low weight average molecular weight polyols are those having a weight average molecular weight of less than 2,000. Similarly, relatively high weight average molecular weight polyols are those having a weight average molecular weight of 2,000 or more.
Preferably, the relatively low weight average molecular weight polyol has a weight average molecular weight of less than about 1,800, more preferably less than about 1,600, and most preferably less than about 1,000.
Preferably, the relatively high weight average molecular weight polyol has a weight average molecular weight of greater than about 2,500, more preferably greater than about 3,000, and most preferably greater than about 3,500. In one embodiment of the invention, the relatively high weight average molecular weight polyol has a weight average molecular weight of greater than about 6,000.
The proportion of the two polyols in such a combination can vary widely. Preferably, however, a majority of the polyol component comprises a relatively high molecular weight polyol. For example, in one embodiment, the relatively low molecular weight polyol comprises about 1 to about 40 percent by weight of the polyol component. In a further embodiment, the relatively low molecular weight polyol comprises about 5 to about 25 percent by weight of the polyol component. Similarly, in one embodiment, the relatively high molecular weight polyol comprises about 60 to about 99 percent by weight of the polyol component. In a further embodiment, the relatively high molecular weight polyol comprises about 75 to about 95 percent by weight of the polyol component.
Isocyanate-Functional Component
The isocyanate-reactive component is reacted with an isocyanate-functional component during formation of the polyurethane-based PSAs of the invention. The isocyanate-functional component may contain one isocyanate-functional material or mixtures thereof. Polyisocyanates, including derivatives thereof (e.g., ureas, biurets, allophanates, dimers and trimers of polyisocyanates, and mixtures thereof), (hereinafter collectively referred to as xe2x80x9cpolyisocyanatesxe2x80x9d) are the preferred isocyanate-functional materials for the isocyanate-functional component. Polyisocyanates have at least two isocyanate-functional groups and provide urethane linkages when reacted with the preferred hydroxy-functional isocyanate-reactive components.
Generally, diisocyanates are the preferred polyisocyanates. Particularly preferred diisocyanates useful in the present invention can be generally represented by Formula II: 
wherein Z represents any suitable polyvalent radical, which may be, for example, polymeric or oligomeric. For example, Z can be based on arylene (e.g., phenylene), aralkylene, alkylene, cycloalkylene, polysiloxane (e.g., polydimethyl siloxane), or polyoxyalkylene (e.g., polyoxyethylene, polyoxypropylene, and polyoxytetramethylene) segments and mixtures thereof Preferably Z has about 1 to about 20 carbon atoms, and more preferably about 6 to about 20 carbon atoms.
For example, Z can be selected from 2,6-tolylene; 2,4-tolylene; 4,4xe2x80x2-methylenediphenylene; 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenylene; tetramethyl-m-xylylene; 4,4xe2x80x2-methylenedicyclohexylene; 3,5,5-trimethyl-3-methylenecyclohexylene; 1,6-hexamethylene; 1,4-cyclohexylene; 2,2,4-trimethylhexylene; or polymeric or oligomeric alkylene, aralkylene, or oxyalkylene radicals and mixtures thereof When Z is a polymeric or oligomeric material it may include, for example, urethane linkages.
The type of polyisocyanate used for the isocyanate-functional material may affect the properties of the PSA. For example, when symmetrical polyisocyanates are used, an increase in shear strength may be observed, as compared to using the same amount of a nonsymmetrical polyisocyanate.
However, any diisocyanate that can react with the isocyanate-reactive material can be used in the present invention. Examples of such diisocyanates include, but are not limited to, aromatic diisocyanates (e.g., 2,6-tolyene diisocyanate; 2,5-tolyene diisocyanate; 2,4-tolyene diisocyanate; m-phenylene diisocyanate; 5-chloro-2,4-tolyene diisocyanate; and 1-chloromethyl-2,4-diisocyanato benzene), aromatic-aliphatic diisocyanates (e.g., m-xylylene diisocyanate and tetramethyl-m-xylylene diisocyanate), aliphatic diisocyanates (e.g., 1,4-diisocyanatobutane; 1,6-diisocyanatohexane; 1,12-diisocyanatododecane; and 2-methyl-1,5-diisocyanatopentane), and cycloaliphatic diisocyanates (e.g., methylenedicyclohexylene-4,4xe2x80x2-diisocyanate; 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate); 2,2,4-trimethylhexyl diisocyanate; and cyclohexylene-1,4-diisocyanate), and other compounds terminated by two isocyanate-functional groups (e.g., the diurethane of tolyene-2,4-diisocyanate-terminated polypropylene oxide polyol). Particularly preferred diisocyanates include: 2,6-tolyene diisocyanate; 2,4-tolyene diisocyanate; tetramethyl-m-xylylene diisocyanate; methylenedicyclohexylene-4,4xe2x80x2-diisocyanate; 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate); 1,6-diisocyanatohexane; 2,2,4-trimethylhexyl diisocyanate; cyclohexylene-1,4-diisocyanate; methylenedicyclohexylene-4,4xe2x80x2-diisocyanate; and mixtures thereof More particularly preferred are 2,6-tolyene diisocyanate; 2,4-tolyene diisocyanate; tetramethyl-m-xylylene diisocyanate; 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate); methylenedicyclohexylene-4,4xe2x80x2-diisocyanate; and mixtures thereof.
Although not as preferred as diisocyanates, other polyisocyanates may be used, for example, in combination with diisocyanates, for the polyisocyanate component. For example, triisocyanates may be used. Triisocyanates include, but are not limited to, polyfunctional isocyanates, such as those produced from biurets, isocyanurates, adducts, and the like. Some commercially available polyisocyanates include portions of the DESMODUR and MONDUR series from Bayer Corporation; Pittsburgh, Pa., and the PAPI series from Dow Plastics, a business group of the Dow Chemical Company; Midland, Mich. Preferred triisocyanates include those available from Bayer Corporation under the trade designations DESMODUR N-3300 and MONDUR 489.
Reactive Emulsifying Compound
When preparing polyurethane-based dispersions of the invention, the isocyanate-reactive and isocyanate-functional components may optionally be reacted with at least one reactive emulsifying compound according to one embodiment of the invention. The reactive emulsifying compound contains at least one anionic-functional group, cationic-functional group, group that is capable of forming an anionic-functional group or cationic-functional group, or mixtures thereof. This compound acts as an internal emulsifier because it contains at least one ionizable group. Thus, these compounds will hereinafter be referred to as xe2x80x9creactive emulsifying compounds.xe2x80x9d
Reactive emulsifying compounds are capable of reacting with at least one of the isocyanate-reactive and isocyanate-functional components to become incorporated into the polyurethane prepolymers. Thus, the reactive emulsifying compound contains at least one, preferably at least two, isocyanate- or active hydrogen-reactive (e.g., hydroxy-reactive) groups. Isocyanate- and hydroxy-reactive groups include, for example, isocyanate, hydroxyl, mercapto, and amine groups.
Preferably, the reactive emulsifying compound contains at least one anionic-functional group or group that is capable of forming such a group (i.e., an anion-forming group) when reacted with the isocyanate-reactive (e.g., polyol) and isocyanate-functional (e.g., polyisocyanate) components. The anionic-functional or anion-forming groups of the reactive emulsifying compound can be any suitable groups that contribute to ionization of the reactive emulsifying compound. For example, suitable groups include carboxylate, sulfate, sulfonate, phosphate, and similar groups.
The incorporation of a reactive emulsifying compound in the polyurethane prepolymer increases water dispersibility of the polyurethane prepolymer. Thus, dispersions prepared from such polyurethane prepolymers have improved dispersion stability as compared to many conventional dispersions. Furthermore, such dispersions may not require external emulsifiers, such as surfactants, for stability.
Preferably, a sufficient amount of reactive emulsifying compound is reacted such that an external emulsifier is not necessary for preparing a storage-stable dispersion. When a sufficient amount of the reactive emulsifying compound is used, the polyurethane prepolymers derived therefrom are also able to be dispersed into finer particles using less shear force than what has previously been possible with many conventional dispersions. A sufficient amount is generally such that the resulting polyurethane-based polymer comprises about 0.5 to about 5 weight percent, more preferably about 0.75 to about 3 weight percent, of segments derived from the reactive emulsifying compound. Below this amount, polyurethanes produced therefrom may be difficult to disperse, and dispersions produced therefrom may be unstable (i.e., subject to de-emulsification and/or coagulation at temperatures above room temperature, or at temperatures greater than about 20xc2x0 C.). However, if polyols containing polyethylene oxide are used, the amount of reactive emulsifying compound used in this preferred embodiment may be less to form a stable dispersion. On the other hand, employing more reactive emulsifying compound in the reaction may produce an unstable dispersion or a resulting PSA that is too sensitive to moisture (i.e., such that physical properties of the PSA are affected to the degree that they are no longer consistently useful for their desired application).
The preferred structure for reactive emulsifying compounds of the invention is generally represented by Formula III: 
wherein Q is a negatively charged moiety selected from COOxe2x88x92 and SO3xe2x88x92, or a group that is capable of forming such a negatively charge moiety upon ionization. Each of X Y, R, and R1 may be the same or different. X, Y, R, and R1 are independently selected from aliphatic organic radicals free of reactive functional groups (e.g., alkylene groups that are free of reactive functional groups), preferably having from about 1 to about 20 carbon atoms, and combinations thereof, with the provisos that: (i.) R can be hydrogen; and (ii.) R1 is not required if Q is COOxe2x88x92.
As an example, dimethylolpropionic acid (DMPA) is a useful reactive emulsifying compound for this embodiment of the invention. Furthermore, 2,2-dimethylolbutyric acid, dihydroxymaleic acid, and sulfopolyester diol are other useful reactive emulsifying compounds. Those of ordinary skill in the art will recognize that a wide variety of reactive emulsifying compounds are useful in the present invention.
Polyurethane-Based Polymer Preparation
In general, the isocyanate-reactive and isocyanate-functional components, along with the optional reactive emulsifying compound, are allowed to react, forming an isocyanate-terminated polyurethane prepolymer (i.e., a polymer having a weight average molecular weight of less than about 50,000). In general, the isocyanate-functional group to isocyanate-reactive group ratio of the reactants is preferably about 1.1 to about 2.5, most typically about 1.5. If the isocyanate-functional group to isocyanate-reactive group ratio is lower than in this preferred range, prepolymer viscosity may be too high to be useful for forming dispersions according to one aspect of the invention.
The isocyanate-terminated polyurethane prepolymer is then chain extended with a chain extending agent (e.g., water (including ambient moisture), a polyamine, a relatively low molecular weight polyol (i.e., a polyol having a weight average molecular weight of less than about 250) and combinations thereof) to increase its molecular weight. When preparing the polymer in a 100% solids system, to chain extend the polyurethane prepolymer, generally the polyurethane prepolymer is first heated to decrease its viscosity.
When preparing the polymer in a waterborne or solventborne system, to chain extend the isocyanate-terminated polyurethane prepolymer, generally the polyurethane prepolymer is first introduced into a dispersing or solvating medium (e.g., water or an organic solvent such as N-methylpyrolidone, acetone, methyl ethyl ketone (MEK), or combinations thereof). The addition of organic solvents in a waterborne system may also help in reducing the viscosity of the prepolymer, which facilitates formation of the dispersion.
In waterborne systems, typically a neutralizing agent is also added to the polyurethane prepolymer to more easily disperse the polyurethane prepolymer in the dispersing medium. For example, a base, such as a tertiary amine or alkali metal salt, can be used as a neutralizing agent to neutralize any anion-forming groups in the polymeric chain and more easily disperse the polyurethane prepolymer in the dispersing medium. Generally, when neutralizing the polyurethane prepolymer before introducing it into the dispersing medium, a tertiary amine is used for the neutralizing agent. When neutralization occurs after introducing the polyurethane prepolymer into the dispersing medium, a tertiary amine, an alkali metal salt, or a combination thereof is used as the neutralizing agent.
In a waterborne system, the polyurethane prepolymer is then chain extended through the reaction of the isocyanate-functional groups in combination with water, at least one polyamine, or mixtures thereof. Isocyanate-functional groups react with water to form an unstable carbamic acid. The carbamic acid then converts to a primary amine and carbon dioxide. The primary amine forms a urea linkage with any remaining isocyanate-functional groups of the polyurethane prepolymer. When the chain extending agent comprises a polyamine, the polyamine forms urea linkages with the isocyanate-functional groups of the polyurethane prepolymer. Thus, the resulting polyurethane-based polymer contains both urethane and urea linkages therein.
As recognizable to those of ordinary skill in the art, the polyurethane prepolymer may alternatively be chain extended using other suitable chain extenders, which may be selected according to whether the polymer is formed using a 100% solids, solventborne, or waterborne system.
When the chain extending agent comprises a polyamine, any suitable compound having at least two amine functional groups can be used for the polyamine. For example, the compound may be a diamine, triamine, etc. Mixtures of polyamines may also be used for the chain extending agent. In general, the isocyanate-functional group to amine-functional group ratio of the reactants is preferably about 0.1 to about 1.5, most typically about 1.
Examples of polyamines useful in the present invention include, but are not limited to, polyoxyalkylene polyamines, alkylene polyamines, and polysiloxane polyamines. Preferably, the polyamine is a diamine.
The polyoxyalkylene polyamine may be, for example, a polyoxyethylene polyamine, polyoxypropylene polyamine, polyoxytetramethylene polyamine, or mixtures thereof. Polyoxyethylene polyamine may be especially useful when preparing the PSA for medical applications, for example, where a high vapor transfer medium and/or water absorbency may be desirable.
Many polyoxyalkylene polyamines are commercially available. For example, polyoxyalkylene diamines are available under trade designations such as D-230, D-400, D-2000, D-4000, DU-700, ED-2001 and EDR-148 (available from Huntsman Corporation; Houston, Tex., under the family trade designation JEFFAMINE). Polyoxyalkylene triamines are available under trade designations such as T-3000 and T-5000 (available from Huntsman Corporation; Houston, Tex.).
Alkylene polyamines include, for example, ethylene diamine; diethylene triamine; triethylene tetramine; propylene diamine; butylene diamine; hexamethylene diamine; cyclohexylene diamine; piperazine; 2-methyl piperazine; phenylene diamine; tolylene diamine; xylylene diamine; tris(2-aminoethyl) amine; 3,3xe2x80x2-dinitrobenzidine; 4,4xe2x80x2-methylenebis(2-chloroaniline); 3,3xe2x80x2-dichloro-4,4xe2x80x2-biphenyl diamine; 2,6-diaminopyridine; 4,4xe2x80x2-diaminodiphenylmethane; menthane diamine; m-xylene diamine; isophorone diamine; and dipiperidyl propane. Many alkylene polyamines are also commercially available. For example, alkylene diamines are available under trade designations such as DYTEK A and DYTEK EP (available from DuPont Chemical Company; Wilmington, Del.).
The polyurethane-based polymer may then be compounded with other materials to form a PSA having the desired properties. That is, PSAs of the present invention may contain various additives and other property modifiers.
For example, fillers, such as fumed silica, fibers (e.g., glass, metal, inorganic, or organic fibers), carbon black, glass or ceramic beadslbubbles, particles (e.g., metal, inorganic, or organic particles), polyamides (e.g., those available from DuPont Chemical Company; Wilmington, Del. under the trade designation, KEVLAR), and the like can be added, generally in amounts up to about 50 parts per hundred parts by weight of the polyurethane-based polymer, provided that such additives are not detrimental to the properties desired in the final PSA composition.
Other additives such as dyes, inert fluids (e.g., hydrocarbon oils), plasticizers, tackifiers, pigments, flame retardants, stabilizers, antioxidants, compatibilizers, antimicrobial agents (e.g., zinc oxide), electrical conductors, thermal conductors (e.g., aluminum oxide, boron nitride, aluminum nitride, and nickel particles), and the like can be blended into these compositions, generally in amounts of from about 1 to about 50 percent by total volume of the composition. It should be noted that, although tackifiers and plasticizers may be added, such additives are not necessary for obtaining PSA properties in polyurethane-based adhesives of the invention.
Application
When the polyurethane-based PSA is prepared from a solventbome or waterborne system, once the solution or dispersion is formed, it is easily applied to a substrate and then dried to form a PSA coating. Drying can be carried out either at room temperature (i.e., about 20xc2x0 C.) or at elevated temperatures (e.g., about 25xc2x0 C. to about 150xc2x0 C.). Drying can optionally include using forced air or a vacuum. This includes the drying of static-coated substrates in ovens, such as forced air and vacuum ovens, or drying of coated substrates that are continuously conveyed through chambers heated by forced air, high-intensity lamps, and the like. Drying may also be performed at reduced (i.e., less than ambient) pressure.
A PSA coating can be formed on a wide variety of substrates. For example, the PSA can be applied to sheeting products (e.g., decorative, reflective, and graphical), labelstock, and tape backings. The substrate can be any suitable type of material depending on the desired application. Typically, the substrate comprises a nonwoven, paper, polymeric film (e.g., polypropylene (e.g., biaxially oriented polypropylene (BOPP)), polyethylene, polyurea, polyurethane, or polyester (e.g., polyethylene terephthalate)), or release liner (e.g., siliconized liner).
PSAs according to the present invention can be utilized to form tape, for example. To form a tape, a PSA coating is formed on at least a portion of a suitable backing. A release material (e.g., low adhesion backsize) can be applied to the opposite side of the backing, if desired. When double-sided tapes are formed, a PSA coating is formed on at least a portion of both sides of the backing.